As a method for producing a silicon single crystal which is used as a substrate of a semiconductor integrated circuit, there is the Czochralski Method (the CZ method), and, in particular, the Magnetic Field applied Czochralski method (the MCZ method) that applies a magnetic field has become prevalent.
In these CZ/MCZ methods, a silicon raw material is charged into a quartz crucible and melted, and a seed crystal is immersed in the raw material melt and then pulled upwardly, whereby a silicon single crystal can be grown. In a single crystal production apparatus (a pulling apparatus) of the CZ/MCZ methods, a heater that heats the raw material melt is placed in a main chamber, and a quartz crucible containing the raw material melt is provided inside the heater.
In general, a raw material is first charged into this quartz crucible, and the raw material is melted by heating by the heater. With the recent increase in the diameter of a silicon single crystal and the length of the crystal, the raw material initially charged into the quartz crucible alone is not enough and a raw material is added in some cases. This is called additional charge by which, as in the case of recharge, which will be described later, the raw material is packed in a recharge pipe having, at a lower end thereof, a valve having the form of a cone (a conical valve) and the raw material is input to the quartz crucible by using the recharge pipe. Then, after all the raw material is melted, the growth of a silicon single crystal is started.
The inside of the quartz crucible is filled with raw material melt which is the molten raw material, and a silicon single crystal is grown therefrom. The grown single crystal is housed in a pull chamber connected to a main chamber upper part via a gate valve and is cooled.
In such production of a single crystal, if only one single crystal is grown from one quartz crucible, the growth of a single crystal is finished at this point of time; however, the quartz crucible is broken and cannot be used again, which increases production costs. Thus, multiple operations by which a plurality of single crystals are grown from one quartz crucible are sometimes performed. In that case, since the raw material melt in the quartz crucible is reduced by an amount corresponding to the grown crystal after the growth of the single crystal, a next single crystal cannot be grown in this state. Therefore, to make up for reduction in the raw material melt, recharge by which the raw material is input again is performed.
As a method of recharge, a rod recharge method, a method of feeding a raw material from a raw material tank as disclosed in Patent Literature 1, and the like have been proposed in the past. However, the technique described in many Patent Literatures is the method of putting a raw material into a recharge pipe having a conical valve at a lower end thereof and inputting the raw material put in the recharge pipe to a quartz crucible. The basics of this technique are disclosed in Patent Literatures 2, 3, and 4.
In such a method, after a silicon single crystal is taken out of a pull chamber partitioned with a gate valve, the recharge pipe in which the raw material is put is attached by being hung with a wire, the gate valve is opened after the inside of the pull chamber is replaced with an argon atmosphere and the furnace pressure is then made to coincide with the furnace pressure in the main chamber, and, after the recharge pipe is moved downward, the opening of the recharge pipe is opened by moving the conical valve downward and the raw material is input.
The raw material which is charged in the manner described above is usually a polycrystal or, in some rare cases, a single crystal and those obtained by crushing these crystals are used, and there are voids between them in a state in which they are packed in the recharge pipe. Thus, to add the raw material corresponding to an amount by which the single crystal was grown, only one input using the recharge pipe sometimes cannot input a sufficient amount of the raw material. In such a case, input of the raw material by the recharge pipe is repeated multiple times.
For example, a method of controlling, during input of a recharge raw material, the positions of a recharge pipe and a conical valve and the position of a quartz crucible into which the input raw material is put is disclosed in Patent Literature 4.